FEMA 453 Design Guidance for Shelters and Safe Rooms

Curtainwall systems are inherently lightweight and flexible
façade systems; however, well designed curtainwall systems
demonstrated, through explosive testing, considerable
resilience in response to blast loading. Furthermore, the glazed
components are subjected to less intense loads as their flexible
supports deform in response to the blast pressures. A multidegree-of-freedom
model of the façade will determine the
accurate interaction of the individual mullions and the phasing
of the interconnecting forces. Because all response calculations
must be dynamic and inelastic, the accurate representation of the
phasing of these forces may significantly affect the performance.
Curtainwall anchors are attached directly to the floor slabs where
the large lateral loads may be transferred directly through the
diaphragms into the lateral load-resisting systems.
Façade systems may contain combinations of glazing, metal
panels, precast concrete, or stone panels. Metal panels provide
little inertial resistance, but are capable of developing large inelastic
deformations. The fasteners that attach these panels to the
mullions or metal studs must be designed to transfer the large
membrane forces. Stone panels provide significant inertial resistance,
but are relatively brittle and have little strength beyond
their modulus of rupture. Stud wall systems that restrain these
façade panels may deform within acceptable levels and develop
a membrane stiffening capacity, and strain energy methods may
be used to calculate their response. However, the anchorage of
the studs to the floor and ceiling slabs are likely to limit the forces
they can develop.
Precast panels may easily be designed to provide inelastic deformation
in response to the design level threats. However, the design of
their anchorage to hold them to the building during both the direct
loading and subsequent rebound phase require more robust
details. Because the primary load carrying elements may buckle in
response to the large collected forces, precast panels are attached
directly to the floor slabs where the forces may be transferred
through the diaphragms to the lateral load-resisting elements.
Where mullions are attached within punched out openings in
2-28 Structural deSign criteria